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Expression and characterization of inactivating and activating mutations in the human Ca2+o-sensing receptor
1Endocrine-Hypertension Division, Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
Abstract:
Nearly 30 mutations have been identified to date in the coding region of the extracellular calcium-sensing receptor (CaR) that are associated with inherited human hypo- and hypercalcemic disorders. To understand the mechanisms by which the mutations alter the function of the receptor may help to discern the structure-function relationships in terms of ligand-binding and G protein coupling. In the present studies, we transiently expressed eight known CaR mutations in HEK293 cells. The effects of the mutations on extracellular calcium- and gadolinium-elicited increases in the cytosolic calcium concentration were then examined. Seven inactivating mutations, which cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, show a reduced functional activity of the receptor because they may 1) reduce its affinity for agonists; 2) prevent conversion of the receptor from a putatively immature, high mannose form into the fully glycosylated and biologically active form of the CaR, in addition to lowering its affinity for agonists; or 3) fail to couple the receptor to and/or activate its respective G protein(s). Conversely, one activating mutation, which causes a form of autosomal dominant hypocalcemia, appears to increase the affinity of the receptor for its agonists.
Insights
Mutations in the calcium-sensing receptor (CaR) cause inherited calcium disorders. This study examined how eight CaR mutations affect receptor function, revealing mechanisms behind hypo- and hypercalcemic conditions.
Area of Science:
- Biochemistry
- Genetics
- Molecular Biology
Background:
- Mutations in the extracellular calcium-sensing receptor (CaR) are linked to inherited human hypo- and hypercalcemic disorders.
- Understanding CaR mutation mechanisms is crucial for discerning structure-function relationships, including ligand-binding and G protein coupling.
Purpose of the Study:
- To investigate the functional consequences of eight known CaR mutations.
- To elucidate the molecular mechanisms underlying inherited calcium disorders caused by CaR dysfunction.
Main Methods:
- Transiently expressing eight known CaR mutations in HEK293 cells.
- Examining the effects of these mutations on cytosolic calcium concentration changes induced by extracellular calcium and gadolinium.
Main Results:
- Seven inactivating CaR mutations, associated with familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, demonstrated reduced functional activity.
- These inactivating mutations may impair agonist affinity, receptor maturation/glycosylation, or G protein coupling/activation.
- One activating CaR mutation, linked to autosomal dominant hypocalcemia, appeared to enhance receptor affinity for agonists.
Conclusions:
- CaR mutations significantly alter receptor function, leading to various inherited calcium imbalances.
- The identified mechanisms provide insights into CaR structure-function relationships and the pathophysiology of calcium disorders.
- Targeting CaR pathways could offer therapeutic strategies for managing hypo- and hypercalcemic conditions.